1. Field of the Invention
The present invention relates to a temperature control apparatus for an exhaust gas sensor capable of detecting the concentration of a specific component in the exhaust gas of an internal combustion engine, and more particularly, it relates to such a temperature control apparatus for an exhaust gas sensor which is adapted to be used with a device having an electric heater arranged in the vicinity of the exhaust gas sensor to activate it at an early stage immediately after the starting of an internal combustion engine, and which is able to heat the exhaust gas sensor so as to promptly raise its temperature to an activation temperature thereof while preventing rapid cooling damage or rapid heating damage (thermal shock damage) of the exhaust gas sensor and/or the electric heater resulting from the splashing or spattering of the condensed moisture (pour water) contained in the exhaust gas during cold starting of the engine.
2. Description of the Related Art
In general, there has been well known the technique that the oxygen concentration in the exhaust gas of an internal combustion engine is detected to control the air fuel ratio (i.e., ratio of air to fuel) of a mixture supplied to the internal combustion engine in a feedback manner, thereby purifying the exhaust gas and improving fuel consumption.
At this case, in order to accurately perform feedback control on the air fuel ratio, it is necessary to keep the temperature of an exhaust gas sensor in an activation range thereof (about 800xc2x0 C.) to stabilize the detection characteristic of oxygen concentration.
Accordingly, in the past, a temperature control apparatus for an exhaust gas sensor has been proposed in which the energization of a ceramic heater built into the exhaust gas sensor is controlled to keep the temperature of the exhaust gas sensor at a constant level (about 800xc2x0 C.).
In addition, it is desirable to rapidly heat the exhaust gas sensor to a proper activation temperature upon starting of the internal combustion engine.
A method for detecting of the temperature of an exhaust gas sensor as a basic technology of such a kind of control apparatus can be seen in, for instance, Japanese patent application laid-open No. 2002-21631 (first prior art document) entitled xe2x80x9cHeater Control Apparatus for the Exhaust Gas Sensorxe2x80x9d.
In the apparatus described in this publication, there are used a method of detecting the internal resistance value of an exhaust gas sensor and estimating the temperature of the exhaust gas sensor itself based on the internal resistance value thereof thus detected, and another method of detecting the ambient temperature of an exhaust gas sensor by measuring the resistance value of a heater built in the exhaust gas sensor.
In addition, Japanese patent application laid-open No. 2002-48763 (second prior art document) entitled xe2x80x9cHeater Control Apparatus for Gas Concentration Sensorxe2x80x9d discloses a method of performing preheating control on a heater according to a reduced voltage during starting of an internal combustion engine so as to avoid rapid heating of the heater at the time of cold starting, thereby preventing damage to elements of the heater due to thermal shock during rapid heating.
Moreover, Japanese patent application laid-open No. 2001-73827 (third prior art document) entitled xe2x80x9cExhaust Gas Sensor and Control Apparatus thereforxe2x80x9d discloses a method of using a double structure protector, a protector heater and a sensor heater in combination so as to prevent damage to elements of the heater due to rapid cooling thermal shock resulting from the splashing or spattering of condensed water at the time of cold starting.
Furthermore, Japanese patent application laid-open No. 2001-41923 (fourth prior art document) entitled xe2x80x9cOxygen Concentration Detection Apparatusxe2x80x9d discloses a means for determining the presence or absence of condensed water and a control means for limiting power supply to a heater in order to prevent damage to elements of the heater due to rapid cooling thermal shock resulting from the splashing or spattering of condensed water at the cold starting of an internal combustion engine.
However, in the above-mentioned first and second prior art documents, there is no mention about pour water rapid cooling thermal shock resulting from the splashing or spattering of condensed water at cold starting, and hence it is impossible to avoid damage to elements of the heater during cold starting.
Further, in the above-mentioned third and fourth prior art documents, no discussion is made about thermal shock due to rapid heating of the heater upon engine starting, and hence it is also impossible to avoid heater element damage due to rapid heating.
Since activating an exhaust gas sensor at an early stage during starting of an internal combustion engine is liable to cause damage to the exhaust gas sensor itself and an electric heater due to thermal shock, as is well-known, it is extremely difficult to achieve the early activation of the exhaust gas sensor as well as the prevention of damage to elements of these sensor and heater (i.e., these are objects giving rise to mutually conflicting problems).
For instance, in the third prior art document, two kinds of heater control are required and hence the apparatus as a whole is complicated in structure, thus leading to an increase in costs.
With the conventional exhaust gas sensor temperature control apparatuses, a variety of improvements have been proposed as referred to above, but they still have a common problem that it is impossible to totally improve the prevention of damage to an exhaust gas sensor and an electric heater as well as the early activation of the exhaust gas sensor.
The present invention is intended to obviate the problems as referred to above, and has for its primary object to provide a temperature control apparatus for an exhaust gas sensor which is equipped with a control means for an electric heater capable of promptly attaining an activation temperature thereof while preventing rapid cooling and rapid heating damage to the exhaust gas sensor and the electric heater.
Another object of the present invention is to provide a temperature control apparatus for an exhaust gas sensor which is equipped with a control means for an electric heater required to prevent the stain or fouling deterioration of the exhaust gas sensor even when it becomes impossible to detect exhaust gas owing to a short circuit, an open circuit, or a break of wiring or the like despite the fact that the exhaust gas sensor itself is operating in a normal state.
Bearing the above objects in mind, according to one aspect, the present invention resides in a temperature control apparatus for an exhaust gas sensor which is mounted on an exhaust pipe of an internal combustion engine for detecting the concentration of a specific component in an exhaust gas therein. The apparatus includes: an electric heater for heating the exhaust gas sensor; a switching element connected in series to the electric heater for turning on and off the supply of electric power to the electric heater; and a temperature control part for performing on/off control on the switching element so as to maintain the temperature of the exhaust gas sensor at a predetermined value. The temperature control part includes: a heating control part for performing heating control on the exhaust gas sensor in accordance with the temperature of the exhaust gas sensor; and a first preheater part and a second preheater for preheating the exhaust gas sensor upon starting of the internal combustion engine. The heating control part includes: a resistance value measurement part for measuring a resistance value of the exhaust gas sensor or a resistance value of the electric heater; a temperature determination part for determining based on the resistance value of the exhaust gas sensor or the resistance value of the electric heater whether the temperature of the exhaust gas sensor is in the vicinity of a proper activation temperature thereof; and a feedback control part for controlling the on/off state of the switching element in accordance with a correlation characteristic of a target temperature and the resistance value of the exhaust gas sensor or the resistance value of the electric heater with the proper activation temperature of the exhaust gas sensor being made as the target temperature. The first preheating part includes a first energization control part for applying a first voltage to the electric heater in a first period. The second preheating part includes a second energization control part for applying a second voltage higher than the first voltage to the electric heater in a second period following the first period. The first and second preheating parts operate prior to the operation of the feedback control part.
According to another aspect, the present invention resides in a temperature control apparatus for an exhaust gas sensor which is mounted on an exhaust pipe of an internal combustion engine for detecting the concentration of a specific component in an exhaust gas therein. The apparatus includes: an electric heater for heating the exhaust gas sensor; a switching element connected in series to the electric heater for turning on and off the supply of electric power to the electric heater; and a temperature control part for performing on/off control on the switching element so as to maintain the temperature of the exhaust gas sensor at a predetermined value. The temperature control part includes; a heating control part for performing heating control on the exhaust gas sensor in accordance with the temperature of the exhaust gas sensor; and a preheating part for preheating the exhaust gas sensor upon starting of the internal combustion engine. The heating control part includes: a resistance value measurement part for measuring a resistance value of the exhaust gas sensor or a resistance value of the electric heater; a temperature determination part for determining based on the resistance value of the exhaust gas sensor or the resistance value of the electric heater whether the temperature of the exhaust gas sensor is in the vicinity of a proper activation temperature thereof; and a feedback control part for controlling the on/off state of the switching element in accordance with a correlation characteristic of a target temperature and the resistance value of the exhaust gas sensor or the resistance value of the electric heater with the proper activation temperature of the exhaust gas sensor being made as the target temperature. The preheating part includes a first energization control part for applying a preheating voltage to the electric heater in a prescribed period, and operates prior to the operation of the feedback control part. The heating control part includes: a sensor line abnormality detection part for detecting an abnormality of the exhaust gas sensor; and a heating keeping part for keeping heating the exhaust gas sensor in response to the sensor line abnormality detection part. The sensor line abnormality detection part generates a sensor line abnormality determination signal upon detection of an open-circuit state or a short-circuit state of a signal line for the exhaust gas sensor. The heating keeping part includes a second energization control part for continuously applying a keeping voltage higher than the preheating voltage to the electric heater upon generation of the sensor line abnormality determination signal.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings.